1. Field of the Invention
This invention relates to reverse osmosis anisotropic membranes obtained from polymeric polyamide materials based on piperazine, and to a process for preparing such membranes.
2. Description of the Prior Art
Membranes having selective permeability have been known for a long time. The principle of reverse osmosis is also known. According to the reverse osmosis principle, when a solution of one or more substances, dissolved in a common solvent, is pumped against a permoselective membrane, at a pressure higher than the osmotic pressure of the solution, it becomes possible to separate the components of the solution.
Moreover, the principle of reverse osmosis has been industrially applied for many years to the desalting of brackish waters and sea water.
In order to effect such a process, it is necessary to utilize membranes capable of allowing the passage of water and of rejecting the salts dissolved therein.
To permit profitable utilization, the reverse osmosis membranes must allow a substantial flow of water therethrough and must also strongly reject the dissolved salts. The membranes suited to such use may exhibit different physical structures. In fact, the polymeric material forming the membrane may have:
(1) a dense and homogeneous structure, either in the form of a supported ultra-thin layer or in the form of a hollow fiber; or PA1 (2) a non-homogeneous structure in the form of "anisotropic gel" (or "skinned") membranes, consisting of a dense and homogeneous superficial polymeric layer generally of 0.1-0.2 .mu. thickness or less, and of a porous understructure which provides a support for the thin layer.
As is known, the high permeability of the anisotropic gel or skinned membranes to the passage of water is due to the thin superficial layer, on which the desalting capacity of the membrane also depends. The term "anisotropic" means that the homogeneous layer, which effects the desalting, is present only on one of the two faces of the membrane.
The polymeric material most widely used for preparing anisotropic membranes is cellulose acetate; in fact, cellulose acetate membranes are those which are generally employed in conventional reverse osmosis processes.
In addition to cellulose acetate, few polymeric materials are known that are capable of providing skinned membranes suited to allow a high flow of water and a strong saline rejection.
U.S. Pat. Nos. 3,687,842 and 3,696,031 both owned by the assignee of this application disclose that polyamides derived from piperazine may be usefully employed for preparing water desalting membranes, according to the reverse osmosis processes.
The polymers described in these patents may be transformed into non-anisotropic membranes of the gel-type which do not, however, exhibit entirely satisfactory characteristics.
Thus, it is an object of this invention to provide reverse osmosis anisotropic membranes based on polypiperazine amides and processes for preparing same which are free from the above mentioned disadvantages.